If you are talking about closing the gap in terms of performance, no: a FF sensor will remain 2.25 times larger then a APS-C one, and when using the same technology in both, the larger surface have some advantages (like when looking at D7000 and D800 which have very similar sensor technology).

I'm not talking about closing the absolute gap in dynamic range. I'm talking about the whole product experience. I guess AF speed and tracking are the main deficits those products have right now.

Huh? Which products are mainly held back by AF speed and accuracy? APS-C or FF cameras?

Mirrorless cameras still don't measure up to DSLRs in terms of focus speed and battery life. DSLR's enjoy a big advantage gap which I believe is much more significant than the difference in sensor performance. But I think that gap will also narrow or become irrelevant eventually.

Actually, I guess the N1 system is already there in good light. I'm sure the others will catch up.

And even when they do shrink the size, they'll differentiate the lens from the more expensive FF version by crippling AF speed.

That part I do not get at all - which lenses focuses slower because they are limited to APS-C? Some cheap lenses focuses slower then more expensive counterparts, but that has nothing to do with sensor size.

The Nikkor 55-300 is, by all accounts, very slow at focusing whereas the 70-300 is much faster. Actually, Nikon's APS-C kit zooms all seem to focus relatively slowly IMO. Whereas the FF lenses are very, very fast in spite of the fact they must move larger mechanical elements. I assume this is an intentional differentiator.